The present invention relates generally to antennas, and more particularly to flat semi-transparent ground planes for reducing multipath reception.
Multipath reception is a major source of positioning errors in global navigation satellite systems (GNSSs). Multipath reception refers to the reception by a navigation receiver of signal replicas caused by reflections from the complex environment in which navigation receivers are typically deployed. The signals received by the antenna in the navigation receiver are a combination of the line-of-sight (direct) signal and multipath signals reflected from the underlying ground surface and surrounding objects and obstacles. Reflected signals distort the amplitude and phase of the received signal. This signal degradation reduces system performance and reliability.
A parameter commonly used to characterize the multipath rejection capability of an antenna is the down/up ratio
            DU      ⁡              (        θ        )              =                  F        ⁡                  (                      -            θ                    )                            F        ⁡                  (          θ          )                      ,where F(θ) is the antenna directional pattern level at an angle θ in the forward hemisphere and F(−θ) is the antenna directional pattern level at the mirror angle −θ in the backward hemisphere. In common practice, the angle θ is the elevation angle measured with respect to the horizon (θ=0° corresponds to the horizon, and θ=90° corresponds to the zenith). To estimate the multipath rejection capability of the antenna, values of DU(θ) over the range of approximately 30°≦θ≦90° are typically used. If the down/up ratio over this angular range is less than approximately −20 dB, the effects of multipath propagation are substantially reduced.
Multipath effects can be reduced by various antenna structures, such as a large, flat ground plane or a ground plane with a choke ring. These structures, however, increase the size and the weight of the antenna. Various other approaches have been developed. As one example, U.S. Pat. No. 6,100,855 discloses a ground plane fabricated from a radar absorbing material that suppresses surface currents on the ground plane and, consequently, reduces reflected signals. This design, however, does not reject multipath signals efficiently; the dimensions, particularly height, are still relatively large for navigation receivers. The radar absorbing material, furthermore, leads to a loss of active power (effective output) and a corresponding decrease in antenna gain.
What is needed is a ground plane with a high rejection of multipath signals, high antenna gain, and compact size.